Hearing loss
The two main types of hearing loss are sensorineural and conductive. Both
types of hearing loss can be present at birth, or they can develop later in
life.
Sensorineural hearing loss (SNHL)
Sensorineural hearing loss occurs because of lack of development or damage to
the special sensory cells for hearing in the inner ear known as hair cells, or
because of damage or lack of full development of the nerve of hearing.
Congenital SNHL
The vast majority of
permanent hearing loss that is present at birth in children is sensorineural.
Approximately half of sensorineural hearing loss in children occurs on a genetic
or hereditary basis. The vast majority (90 percent) of children with
sensorineural hearing loss that has occurred on a genetic basis have two parents
with normal hearing. This situation arises because most genes for sensorineural
hearing loss are recessive. Parents with a single recessive gene for hearing
loss have normal hearing themselves because they also carry another unaffected
gene.
However, if each hearing parent has one gene for hearing loss as well as a
gene for normal hearing, their child may acquire two genes for hearing loss (one
from each parent). Fortunately, most children with sensorineural hearing loss
that has occurred on a genetic or hereditary basis are otherwise healthy. These
children have what is called nonsyndromic sensorineural hearing loss. A gene
called Connexin 26 has been identified that is estimated to be the underlying
cause of half of nonsyndromic sensorineural hearing loss.
A smaller number of children have a syndrome in which the hearing loss is
present in association with another physical finding or medical problem. The
following are several examples of syndromes associated with sensorineural
hearing loss that occur on a genetic basis:
- Waardenburg's
- Jervell and Lange-Nielsen
- Pendred's
- Usher's
Sensorineural hearing loss can also be present at birth because the mother
experienced certain types of infections during pregnancy, for example,
toxoplasmosis, rubella, cytomegalovirus (CMV), syphilis.
It is estimated that 50 percent of sensorineural hearing loss present at
birth does not occur on a genetic basis. No identifiable cause can be found in
more that half of these non-genetic cases.